The present invention relates generally to optical lenses. More particularly, the present invention relates to optical retro-reflectors.
Retro-reflective surfaces prove useful in a variety of applications, including projector screens, highway signs, road markings, safety marking, and surveying instruments. For example, by modifying the retro-reflectance properties of a projector screen, brighter images can be obtained at desired viewing angles without increasing the output power of the projector.
One known way to provide a retro-reflective surface is to coat the surface with a large number of small glass spheres or “micro-spheres.” These micro-spheres typically have dimensions of about 1 micrometer to 50 micrometers, and a refractive index relatively high as compared to air. The micro-spheres are typically attached to the surface using a reflective binder. The performance of micro-sphere based retro-reflectors can be poor, however, due to low reflection efficiency or limited viewing angles. Although a theoretically ideal spherical lens, known as a Luneburg lens, can provide excellent properties, practical fabrication of such a lens has proven elusive.
One problem with projector screens is that the screen can reflect both the desired projected light and undesired ambient light. Unfortunately, there can be tradeoffs between providing high reflectivity (sometimes called “screen gain”), wide viewing angle, and ambient light rejection (sometimes called “contrast”). Various manufacturers have gone to great lengths to improve these various screen particles. For example, one approach uses a combination of spherical and ellipsoidal particles to enhance the viewing angle.
Hence, what is needed is an improved technology for providing a highly reflective surface with desired retro-reflectivity properties.